Color projection display apparatuses breakdown each of the three primary color red, green and blue light components from white light and lead those components to the respective corresponding spatial light modulating elements, where the color lights modulated in coordination to an image signal are synthesized and projected, causing color images to be displayed on screen.
Systems using reflective type spatial light modulating elements for a projection display apparatus are beneficial for producing high resolution, however there is a tendency for such apparatuses to have complex optical structures. A projection image display apparatus is disclosed in Japanese Patent Application Laid-Open Publication No. 2001-174755, pp. 8–9, FIG. 8.
Japanese Patent Application Laid-Open Publication No. 2001-174755 discloses a structure comprising a lens for focusing light of the three primary colors, already made into p-polarized light as light of indeterminate polarity passes a polarizing panel or the like, a dichroic mirror, inclined at a 45° angle to the optical axis, that reflects green p-polarized light from this white light source and passes other p-polarized lights, a primary polarized light beam splitter prism that passes green p-polarized light reflected at the dichroic mirror and reflects s-polarized green light, a wavelength selective phase plate for red light, that rotates the plane of polarization of red light among the other p-polarized lights passing the dichroic mirror 90° and passes p-polarized blue light, and a secondary polarized light beam splitter prism that reflects red light, made into s-polarized light at the wavelength selective phase plate for red light and passes p-polarized blue light.
Further, the system comprises, spatial light modulating elements for green light, operating on the side of the primary polarized light beam splitter prism of passed green light, that modulate p-polarized green light in relation to an image signal, forming that light into s-polarized light and reflecting it, spatial light modulating elements for red light, operating on the side of p-polarized red light reflected at the secondary polarized light beam splitter prism, that modulate s-polarized red light in relation to an image signal, forming that light into p-polarized light and reflecting it, spatial light modulating elements for blue light, operating on the side of the secondary polarized light beam splitter prism of passed p-polarized blue light, that modulate p-polarized blue light in relation to an image signal, forming that light into s-polarized light and reflecting it, and a wavelength selective phase plate for blue light, that rotates 90° the plane of polarization of blue light from among p-polarized red light passing the secondary polarized light beam splitter prism and s-polarized blue light reflected at the secondary polarized light beam splitter prism, making that blue light into p-polarized light and passing p-polarized red light.
Moreover, that system comprises a tertiary polarized light beam splitter prism that reflects s-polarized green light reflected at the primary polarized light beam splitter prism and passes p-polarized blue and red lights passing the wavelength selective phase plate for blue light, and a projection lens for projecting p-polarized red, green and blue lights emitted from the tertiary polarized light beam splitter prism.
Further, that system comprises a λ/4 phase plate disposed between the primary polarized light beam splitter prism and the spatial light modulating elements for green light, that forms p-polarized green light emitted from the primary polarized light beam splitter prism into linearly polarized light and linearly polarized light from the spatial light modulating elements for green light into elliptically polarized light without green light reflected at the spatial light modulating elements for green light being slantingly injected into the primary polarized light beam splitter prism and suffering polarity degradation.
A secondary λ/4 phase plate the same as the above-mentioned plate is disposed between the secondary polarized light beam splitter prism and the spatial light modulating elements for red light, and a tertiary λ/4 phase plate the same as the above-mentioned two plates is disposed between the secondary polarized light beam splitter prism and the spatial light modulating elements for blue light.
The optical structure resulting from this projection display apparatus is simple and it is difficult for light beams to become mixed, thereby enabling a projection image having high contrast to be obtained at low cost.
The polarized light separating surfaces used in the primary, secondary and tertiary polarized light beam splitter prisms are formed of an optical membrane of tens of dielectric membranes deposited on the inclined face part of a right angle prism, as shown in FIG. 1A and FIG. 1B, the angle of incidence being a parameter for the rate of permeability for this optical membrane.
FIG. 1A shows wavelength dependence of the rate of permeability p-polarized light in visible wavelength regions when the angle of incidence β of incident light L1 to the transmittance face of a polarized light beam splitter prism 30 is a parameter as shown in FIG. 1B.
FIG. 1A shows the cases for the angle of incidence β of light incident L1 to the transmittance face of the polarized light beam splitter prism 30 where ‘a’ is 0°, ‘b’ is 6°, ‘c’ is 15°, ‘d’ is +6° and ‘e’ is +15°.
The angle of incidence β is the angle where incident light L1 to the polarized beam splitter prism provides the optical axis 30.
As shown in FIG. 1A and FIG. 1B, when the angle of incidence β of light incident L1 to the transmittance surface of a polarized light beam splitter prism 30 is within ±6° wavelength dependence of the rate of permeability of p-polarized light is relatively uniform, however when this angle of incidence is exceeded there is substantial wavelength dependence and the rate of permeability deteriorates.
Normally, because main light source input parallel with the optical axis or light incident at angles other than the angles prescribed input to a polarized light separating surface are included in light of indeterminate polarity, even if a main light beam is input to the polarized light separating surface parallel with the optical axis, light other than that main light cannot be input parallel with the optical axis. Thus, as described above, when light of indeterminate polarity is input to the polarized light separating surfaces color reproducibility deteriorates because permeability at those polarized light separating surfaces differs according to wavelength.
As a solution to this, it is possible to consider making the angle of incidence of light of indeterminate polarity input to the polarized light separating surfaces narrower, reducing the wavelength dependence of the rate of permeability, however this causes light of indeterminate polarity other than such light to become external light which is not used by the projection display apparatus, resulting in deteriorating brightness.
Further, heat distribution arises as light of the three primary colors is absorbed by the primary, secondary and tertiary polarized light beam splitter prisms, the refractive index changing due to differences of temperatures in the primary, secondary and tertiary polarized light beam splitter prisms and double refraction occurs leading to deteriorating image quality. This is because light polarizing surfaces within the prisms forming a polarized light beam splitter prism partially rotate. In order to prevent these problems from occurring it is necessary to use material having a low photo elasticity factor to suppress the double refraction of glass material used, however this is more costly.
Moreover, as it is becoming desirable for projection display apparatuses to be increasingly miniaturized, these polarized light beam splitter prisms, being the heaviest component within the apparatus, must be made lighter. Again, as the size of display elements increases the size of these prisms increases also, making these prisms increasingly weighty.
With the foregoing in view it is an object of the present invention to provide a lightweight projection display apparatus producing high-quality images.